Cured gels including silicone gels have been suggested for a wide variety of applications to environmentally seal and protect substrates such as electrical connections, bare electrical conductors and splices and in other non-electrical applications such as the protection of metallic structures from corrosion, for example, by sea-water.
Cured silicone products having a wide range of properties useful in a wide range of applications are well known, as is the formation of those products by curing organopolysiloxanes by crosslinking individual organopolysiloxanes or mixtures of various organopolysiloxanes. One particular family of organopolysiloxanes, for example, which are useful in forming adhesives and other materials are those diorganopolysiloxanes which are terminated at either or both ends of the chain with vinyl groups. Examples of such vinyl terminated polysiloxanes appear in U.S. Pat. No. 4,196,273 to Imai et al. Vinyl terminated polysiloxanes may be crosslinked with themselves or with other polysiloxanes containing various functional groups such as aryl, aliphatic (saturated or unsaturated), and fluoroaliphatic moieties (such as CF.sub.3 CH.sub.2 CH.sub.2 --) or containing other groups such as nitrogen groups, sulphur groups and the like. Examples of such organopolysiloxanes are shown in U.S. Pat. Nos. 3,624,022 to Ross, 4,064,027 to Gant, 4,163,081 to Schulz and 3,445,420 to Kookootuedes. Diorganopolysiloxanes terminated at the chain ends with two or three vinyl groups have been crosslinked to provide elastomeric or rubber-type products, as shown in U.S. Pat. No. 4,364,809 to Sato et al.
Commonly assigned copending U.S. application Ser. No. 730,402 filed May 2, 1985, now U.S. Pat. No. 4,777,063, the disclosure of which is completely incorporated herein by reference for all purposes, discloses novel curable organopolysiloxane compositions comprising at least about 50% by weight of a general organopolysiloxane having no more than two vinyl groups and up to about 50% by weight of a crosslink additive which is a polysiloxane having at least 3 vinyl groups and which is copolymerizable with the organopolysiloxane. This curable composition may be cured by heat, chemical or, preferably, radiation curing techniques to produce crosslinked polysiloxane materials.
In recent years, as disclosed inter alia in U.S. Pat. Nos. 4,600,261 and 4,634,207 (Debbaut), articles comprising an apparatus for protecting a substrate, including electrical contacts and comprising a gel of recited properties, the gel being cured prior to coming into contact with any part of the substrate to be protected, and means for deforming the gel into close and conforming contact with the substrate have found with acceptance as a particularly effective means for sealing and environmentally protecting substrates, especially when the article may be disengaged from the substrate such that the gel remains substantially within said apparatus. Especially with electrical contacts (for example, terminal posts) such as are used by telephone companies to connect the telephone line twisted pair to the line going to an individual household and which contacts are customarily placed inside a Pedestal box mounted at about ground level and exposed to the elements, sealing and encapsulating materials and articles used to protect and insulate said contacts are therefore subjected to temperatures in the range from about -40.degree. to about 60.degree. or even 70.degree. C., to insect damage, to water (both liquid and vapor) and must have means to enable the telephone company artisan to reenter the box and alter or repair contacts, connections, splices and wires contained therein. Such connecting means (indeed connectors generally and terminal posts in particular) are usually of high aspect ratio such that the above mentioned cured gel must be capable of withstanding considerable and repeated deformation as the protection apparatus is applied, removed and applied again. It is essential also that the gel, on removal leave no significant amount still adhering to the connector or to the bare electrical connectors attached thereto. Thus it is highly desirable that cured gels useful in these and similar applications possess excellent physical properties, including high elasticity and elongation to provide high deformability; high toughness (a combination of high elongation and tensile strength), a cohesive strength greater than its adhesive strength and greater adhesion to the apparatus containing it than to the substrate to ensure the gel remains substantially within the apparatus when it is removed; excellent tack, adhesive properties, resistance to stress relaxation and low compression set to prevent water ingress along the interface between the gel and the bare electrical conductors or connector; good stability to syneresis under compression (as hereinbelow described) to prevent shrinkage of the gel and contamination of its environment; high hydrolytic, thermal and oxidative stability; that it be moisture insensitive and that it possess excellent resistance to the damaging effects of ultra-violet (u. v.) light to enable it to survive exposure to the elements for the long service life contemplated for such devices by telephone companies.
The polyurethane gels used by the prior art have excellent short term physical properties but are poor in such long term chemical and physical properties as hydrolytic and thermal stability, resistance to moisture and resistance to compression set especially at higher temperatures within the anticipated service range (hereinafter collectively described as aging resistance). Silicone gels used by the prior art have poor physical properties, particularly low elongation and toughness; but have good thermal stability and excellent moisture insensitivity if formulated appropriately, for example, by keeping inadvertently or deliberately added ionic species to a minimum. Because of their poor physical properties, pre-cured silicone gels are not preferred materials for use in applications where encapsulation of high aspect ratio substrates, such as the hereinabove mentioned terminal posts, and/or removal and replacement of protective apparatus is required. Thus there is a widely felt need for a cured gel with excellent physical properties as described hereinabove and good thermal, hydrolytic, u. v. and oxidative stability especially a need for gels which are also moisture insensitive.